1. Field of the Invention
The present invention relates to an improvement of a bearing apparatus which is to be used for bearing a rotation shaft of, for example, a rotor of an electric motor.
2. Description of the Prior Art
A conventional bearing apparatus used in an electric motor is described referring to FIG. 4. FIG. 4 shows only the part of the conventional oilless bearing of an electric motor. A rotor and a stator of the motor are not shown in the figure for the purpose of simplifying the explanation.
In FIG. 4, a rotation shaft 1 of a rotor 8 of the motor (not shown in the figure) is rotatably born by oilless bearings 2 made of sintered oilless metal. In the vicinity of an end 1a of the shaft 1 (lower part in the figure), a washer 3 is provided for receiving a thrust load of the shaft 1 by sliding on a thrust bearing surface 2a of the oilless bearing 2. A stopper 4 is provided on the shaft 1 for making the washer 3 contact the thrust bearing surface 2a of the bearing 2, stably. The oilless bearings metals 2 are supported by a bearing holder 5 which is made of an insulative material. A sealing cap 6, which is for sealing lubricant oil so as not to leak out of the bearing holder 5, is provided on an opening 5a of the bearing holder 5. An oil thrower 7 is provided on another end 1b of the shaft 1 for pushing back flow of the lubricant oil from the oilless bearings 2.
Operation of the above-mentioned conventional bearing apparatus is described.
At first, when the shaft 1 is rotated, outer surface 1c of the shaft 1 slides on an inner bearing surface 2b of the oilless bearings 2. And thereby, temperature of the oilless bearings 2 is increased by friction heat due to the sliding of the shaft 1 on the inner bearing surface 2b of the oilless bearings 2. Next, the lubricant oil impregnated in the oilless bearings 2 is expanded and the viscosity of the lubricant oil is decreased by the friction heat. As a result, the lubricant oil gushes on the inner bearing surface 2b of the oilless bearings 2. The rotation of the shaft 1 is lubricated by the gushed lubricant oil so as to decrease the friction resistance between the shaft 1 and the oilless bearings 2 to a low level, so as to avoid direct contact between the metallic materials of the shaft 1 and the oilless bearings 2. Thus, the wear of the shaft 1 and/or the oilless bearing 2 is decreased, thereby to maintain the property of the oilless bearings 2 in a long service.
When the above-mentioned conventional bearing apparatus, however is used in an enviroment at a high temperature, the lubricant oil further expands and the viscosity of the lubricant oil is further decreased. Thereby, the lubricant oil which can not be held by the oilless bearings 2 overflows to the spaces in the vicinity of the oil thrower 7 and the sealing cap 6 in the bearing holder 5. The lubricant oil having overflowed in the space in the vicinity of the sealing cap 6 does not leak out from the bearing holder 5, since the sealing cap 6 serves as a leak-prevention member against the opening 5a of the bearing holder 5. The inner pressure of the bearing holder 5, however, increases because of the leak-prevention function of the sealing cap 6. Thereby the inner pressure of the oil pushes the lubricant oil having stayed in the space in the vicinity of the oil thrower 7 to the outside of the bearing holder 5. By such an increase of the inner pressure, the lubricant oil overflows from the bearing holder 5 to the outside thereof, and thereby the service life of the oilless bearings 2 is shortened.
Generally, the electric motor repeats the revolving and the stop. The oil thrower 7 serves for pushing back the flow of the lubricant oil to the oilless bearings 2 during the rotation of the rotation shaft 1. However, when the rotation shaft 1 stops its rotation, the oil thrower 7 does not serve the prevention of the leak out of the lubricant oil at all. Accordingly, the lubricant oil, except at a part which is held by the surface of the oilless bearings 2 by surface tension, leaks out from the bearing holder 5. And thereby the service life of the oilless bearings 2 is shortened.